In the context of this invention, the term "composite" describes a material consisting essentially of high strength fibers or filaments of graphite, or other material, embedded in a matrix of a thermosetting resin which serves when cured to maintain the alignment of the fibers and their relationship to one another within the matrix as the material is stressed. As applied to the construction of aerodynamic surfaces, composite material has heretofore taken the form of woven mats preimpregnated with resin and, more recently, resin tapes embedded with fibers or filaments aligned in the longitudinal direction of the tape, multiple courses of which are laid side by side to construct one ply or layer of a manufactured article which is then constructed incrementally of successive layers of tape.
The application of these construction techniques to contoured surfaces heretofore has been essentially one of first laying up the laminated structure on a flat surface and then transferring or pressing the layup into a mold having the final contour of the part to be fabricated so that the layup will assume the desired shape. The mold with the composite layup applied thereto is then autoclaved. The layers of resinous matrix material merge into a unitary structure during the initial stages of the process and then solidify upon continued exposure to the high temperature in the autoclave as the resin cures.
The described system poses a number of problems in the molding of surfaces of compound curvature i.e., those curved in multiple planes or on multiple axes. One problem is conforming a plane table layup to the compound surface of a mold and this problem becomes more prominent with the severity of the curvature encountered. In all cases, irrespective of curvature, the mere necessity of transferring the layup from a flat lay surface to a mold and pressing it into conformity with the mold surface is a labor-intensive and time-consuming operation.
It is accordingly desirable from the standpoint of manufacturing efficiency and the integrity of the final product to form the laminated layup with composite tape laid directly upon the compound surface of an appropriately shaped tool or mandrel. This is preferably done with composite tape to make the most efficient use of the strength of the fibrous material, as well as to conform the essentially planar form of the building material more readily to the compound curvature of the mandrel. Such conformance is much more easily accomplished with composite tape than by the use of broad goods.
Even with composite tape, however, the practical necessity of working with tapes of finite widths in the range of from 1 to 6 inches and thicknesses of about 0.0055 to 0.010 inches, and the essential inelasticity of the fiber core of the tape under laying conditions, create their own problems. The primary difficulty is conforming the composite tape to the compound surface without puckering one edge or the other of the tape as the tape laying mechanism follows, within limits, the curvature of the lay surface on any selected tape course.
To overcome this problem, it has been proposed, as disclosed in U.S. Pat. No. 4,696,707, entitled "COMPOSITE TAPE PLACEMENT APPARATUS WITH NATURAL PATH GENERATION MEANS" and issued to Lewis et al., which is commonly assigned with this invention, that any given lay surface first be defined mathematically with respect to the tape laying machine coordinate system, and that the tape be applied to such a surface by following a preprogrammed natural path of the tape thereon while conforming as nearly as possible to the direction in which the designer would prefer to have the fibers aligned for the sake of the strength of the part. By the technique disclosed in Lewis et al., the path of each successive course within a ply, and each successive ply in the layup of the laminated article, is predetermined such that the machine is programmed to lay the tape without tensioning the tape edges unequally. This avoids the puckering of the tape along either of its edges as would inevitably happen if the laying mechanism sought to steer the tape forcibly to any substantial degree away from its natural path.
To rapidly and accurately generate a natural tape path for a part program, the programmed machine of Lewis et al. uses a mathematical description of the lay surface on the workpiece area of a mandrel forming the complex contoured shape. This mathematical description of the lay surface describes the shape and contours of the mandrel in terms of various Z-axis heights or offsets from a control plane broken into convenient X-Y areas. Because it is the tape laying head of the machine which is to be moved, the X-Y areas of the control plane are referenced not to the actual coordinates in space of the surface of the mandrel but to the internal coordinate system of the tape laying machine. Therefore, there is at least one area of concern in laying tape precisely on the mandrel which must be addressed to enhance the rapid manufacture of parts in this manner.
The problem which must be overcome for the precision production of layered parts is that the mandrel may not exactly conform to the mathematical description used to generate the natural path part program. There are many reasons for a discrepancy between the actual surface and the mathematical description thereof. Some of the most prevalent reasons include an approximation of the surface in the first instance, the inability to manufacture two mandrel surfaces exactly the same, and changes in the mandrel surface due to tape laying environment such as wear, warpage, etc.
When making the mathematical description of a mandrel surface in the first instance, the description of the actual surface must be an approximation to some degree. The more surface points that the description uses the better the approximation will be, but it is still an approximation. Moreover, if one uses less points, some sacrifice in accuracy will be seen but at the gain of production rates as less surface points have to be accounted for by tape head movement. Because of the size and complexity of the mandrels used in the production process, no two can have exactly the same surface. An ideal or designed mandrel surface, a standard, is used to make the mathematical description by digitization or the like. The amount by which other actual mandrels differ from this standard will cause errors in the tape laying process. However, this process allows mandrels to be interchangeable from part to part and from production facility to production facility.
Moreover, each mandrel surface will change in the ordinary course of production over time because of different environmental conditions such as expansion or contraction, wear, and warpage. Additionally, when making a part of many layers, the mathematical description assumes that each ply is a constant thickness and changes the description accordingly. The composite tape on an average is a nominal thickness but each ply is different and changes the actual surface from the original representation by a small amount. Although each actual change may be extremely small, some changes tend to accumulate and with the use of the same part program for different mandrels, there may be changes that occur in different directions which exacerbate an attempt for remedy.
Adaptive control has been suggested as a solution to the problem of deviations of the actual surface from the representative surface. Previous attempts to lay composite tape on a complex contour by seeking to control the lay path direction completely by adaptive sensing of the tool surface have encountered significant problems.
One problem with prior adaptive approaches has been the selecting of control gains and authority levels for the control. If the control is made sensitive enough to respond to slight variations in surface imperfection, then, at the velocities and gains needed for this precision, the control becomes oscillatory and unstable and will not settle readily into a smooth motion such that composite tape can be correctly laid. Alternatively, if the gain and authority of the control is reduced significantly, then the lack of precision that the tape layer displays in following a correct path fails to produce the desired quality in the parts. The only ready answer to this problem is to slow the machine to production rates which are unacceptable, i.e., to trade off productivity for control.
Further, for multi-axis tape laying machines simultaneous adaptive control of all axes has not been advantageous. The coupling of all the adaptive inputs into one control algorithm has proven unworkable because the interaction between separate axes is not readily definable. Without a precise definition of the complex coupling mechanisms between movements, an integrated adaptive control cannot achieve the precision necessary for tape laying machines.
Another problem which is inherent in the previous adaptive approaches and which cannot be readily solved is the basic nonreproducibility of the parts. Because prior adaptive controls continue to hunt along a tape course path and do so in an unpredictable manner, every part made by this method, even on the same tool surface, will be different from any other part. Therefore, quality control and reproducibility of precision parts has not been obtainable by adaptive methods.
The natural path control set forth in Lewis et al. solves the quality control problem by precisely and reproducibly generating a programmed path for a composite tape laying head which will produce complex contoured parts with facility. Because it is desirable to retain the exact tracking path from part to part, deviations of the actual surface from the mathematical description which is used to calculate the natural path are not compensated by the part program.
It is now known that differences between the mathematical representation of the mandrel surface and the actual surface tend to cause more problems in the area of the quality of the tape lay, i.e., whether the tape course is properly adhered to the underlying surface, than in course path tracking. Therefore, adaptive controls which do not affect the tracking path but affect only the tape laying conditions can provide gains in the quality of the tape laying. The correct laying conditions for composite tape are a substantially constant pressure on the tape with the pressure being applied by a roller surface substantially flat against the contour of the tool surface, and with the tape directly under the roller surface. An adaptive control system which can correct for actual variations in the tool surface without changing the natural tracking path of the machine would be advantageous because the reproducibility and speed of the natural path control would be retained while insuring that the composite tape was laid under the optimum conditions.
The adaptive control system of the present invention is adapted to be superimposed upon a preprogrammed natural path for a tape laying head for the sake of dealing with discrepancies between the part program and the actual lay surface as it is encountered by the tape laying head in action. Such discrepancies, depending upon their nature, may call for independent local or transient adaptive adjustments of the tape laying head on one or more of its axes of movement without deviating from the natural tracking path.